Such a tractor mechanism is known, inter alia from the U.S. Pat. No. 3,825,162. For advancing a paper web in one direction through a printer there are two tractor mechanisms of this kind arranged with their respective holders movable on two parallel, horizontal rods, of which one is a drive shaft. More specifically, the toothed belt in each tractor mechanism runs over two rollers mounted in the holder, the drive shaft being in driving engagement with one of the wheels by the above-mentioned drive connection. The printer contains a motor which rotates the drive shaft.
Of the holders, the grooved wall portion is situated on a pivotable lid, the gap and the portion of the toothed belt running through it being uncovered by raising the lid. With the lids pivoted the paper web is brought in between the tractor mechanisms with both its holed edges in engagement with the toothed belt of the respective mechanism, subsequent to which the lids are repivoted. The paper is then securely held for advancing through the gaps in the tractor mechanisms.
A problem with printers provided with tractor mechanisms of the kind described above is that the mechanisms in certain cases may be located comparatively inaccessibly in the printer, so that inserting the paper by pivoting up the lids is made difficult. This problem is particularly apparent in types of printers where each tractor mechanism has two mutually opposing gaps on either side of the holder, these gaps being of the kind mentioned above with associated lids, the paper web running through both gaps and thus engaging twice with each toothed belt. The advantage of this type of paper feeding is that it facilitates movement of the paper web in the reverse direction. With relation to insertion of the paper, however, one of the gaps is usually situated considerably more inaccessibly than the other, e.g. under it.